(In [15413]) Add keybindings for jumping to the top, middle, and bottom of the title
and recordings lists on the Watch Recordings screen.

The patch in ticket #4451 hardcoded these actions to 2, 5, and 8 respectively,
but I have modified it to allow the user to bind these actions to any key.
The keybindings are PAGETOP, PAGEMIDDLE, and PAGEBOTTOM.

I have the dubious honor of reporting a bug for code that I (originally) wrote. The PAGEBOTTOM and PAGEMIDDLE bindings don't work in the "virtual list" Watch List, Categories, Recording Groups, or Searches. They only work in vanilla recording lists, where iterating progIndex from 0 to (progLists[titleList[titleIndex]].count() -1) always points to a recording that appears in the list.

I did not discover the bug earlier as a) the patch was originally written against 0.20, which doesn't have Watch List, and b) in 0.21 I normally don't turn anything in Group View on except Titles and Watch List. I presume no one else has yet reported the bug because of reason b) and because PAGETOP, PAGEMIDDLE, and PAGEBOTTTOM don't have default keybindings so most people haven't discovered them yet.

Unfortunately, I don't yet understand how the virtual lists are constructed--writing the original patch tested the limits of my C++ skills, such as they are--so I hope someone else can fix the issue.

This commit still doesn't add default keybindings for these since we have
been trying not to add new defaults for optional items such as this because
of the large number of existing actions and jumppoints that already exist.
We prefer to leave optional actions like this up to the user to bind rather
than risk potential conflict with an existing binding which the user may have
configured.